handbook of carbon, graphite, diamond and fullerenes

20 Carbon, Graphite, Diamond, and Fullerenes
chemical or physical conditions of the atom. Disintegration of a radioactive
isotope is measured in terms of half-life, which is the time required for the
original number of radioactive isotopes to be reduced to one-half.
As shown in Table 2.4, the 10 C, 11 C, 15 C and 16 C isotopes have short
half-lives, and their practical use is therefore limited. On the other hand, 14 C
has a long half-life and is a useful isotope with important applications (see
below).
The atomic structures of 12 C, 13 C and 14 C are shown schematically in
Fig. 2.2.
(Stable)
13C
(Stable)
14c
(Radioactive)
Figure 2.2. Major carbon isotopes. Note equal number of protons but different
number of neutrons.! 8 '
2.2 Carbon Dating with Carbon-14
The radioactive decay of 14 C and of other radioactive isotopes, such
as uranium-235 and -238, thorium-232, rubidium-87 and potassium-K40,
provide a reliable way of dating materials. Carbon-14 can only be used to
date carbonaceous compounds. Its long half-life of 5730 years permits
accurate dating for up to 30,000 years. This period is approximately equal
to five half-lives, after which only 1/32nd of the original amount of 14 C
remains, which is no longer sufficient to permit precise measurements.^ 9 '
Mechanism of Formation and Decay of 14 C. The chemist WillardF.
Libby discovered in 1946 that 14 C is continuously being formed in the earth's
atmosphere by the reaction of the major nitrogen isotope, 14 N, with highly
energetic neutrons originating as a secondary radiation from cosmic rays. [9]
In this reaction the 14 N atom gains a neutron (going from seven to eight) and